For centuries grinding mills have been used to break up solid materials into smaller pieces. Some of the largest grinding mills today are used in the cement and mining industries. These impressive mills can reach up to 40 feet (13.4 meters) in diameter with 28 mega-watt gearless mill drives (“GMD”). Such mills provide high throughput and round-the-clock operability for meeting the world's ore and cement demand.
The mechanical components that make up the gearless mill drive (“GMD”) in grinding mills, such as the rotor poles, flanges, support ribs, and welds are highly susceptible to fatigue and crack propagation. This is due, in part, to cyclic tensile loading caused by gravitational forces and fluctuating magnetic forces. Cracks in mill drive components can lead to costly repairs and lengthy downtime, causing a mill plant substantial financial loss.
The problem of fatigue and crack propagation in GMDs is discussed in more detail in the following papers: “Problem Definition And Repair Of The Rotor Pole Structure On One Of The World's Largest Gearless Drive SAG Mills,” by Phil Gunn, SAG 2006; and “Remedial Design Of The World's Largest SAG Mill Gearless Drive,” by Meimaris, Lai, & Cox, SAG 2001.
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Those of skill in the art have proposed numerous solutions to the problem of fatigue in GMDs. Some of the solutions include: (1) increasing the number of support ribs; (2) increasing the height of the support ribs; (3) adding supporting gussets to the ribs; (4) optimizing the shape of the ribs; and (5) improving weld quality. Unfortunately, these solutions fail to account for the underlying cause of crack propagation: cyclic tensile loading on the weld seams between the ribs and flanges. In addition, these solutions tend to increase the GMD's diameter, weight, and cost.
Another proposed solution is to make the ribs one solid piece that is disposed in an opening of the flange. While this approach addresses the underlying cause of crack propagation by minimizing cyclic tensile loading on the flange-rib seam, the design continues to rely on welds for connecting the ribs to the flanges, which is highly susceptible to fatigue.
It has yet to be appreciated that support ribs for GMDs can be weld-free. Furthermore, it has yet to be appreciated that support ribs for GMDs can couple with flanges and rotor poles via mechanical engagements to improve fatigue resistance. Thus, there is still a need for improved designs for supporting ribs in GMDs.